Battery Life Calculator
Estimate how long a battery lasts from its capacity (mAh) and the device load current (mA), with an adjustable efficiency factor.
Result
- Runtime (human-readable)17h 0m
- Ideal runtime (100% efficiency)20.00 hours
- Efficiency factor applied85%
- Usable capacity2,550 mAh
- C-rate (discharge rate)0.05 C
Step-by-step
- Charge relation: a battery rated 3,000 mAh sustains 1 mA for 3,000 hours, so runtime = capacity ÷ load.
- Apply the efficiency factor to the rated capacity: usable = 3,000 mAh × 85% = 2,550 mAh.
- Divide usable capacity by the load: 2,550 mAh ÷ 150 mA = 17.00 hours.
- That is about 17h 0m.
How to use this calculator
- Enter the battery capacity in mAh (printed on the cell, e.g. 3000 for a typical 18650).
- Enter the average current your device draws in mA — use the typical running draw, not the peak.
- Set the efficiency factor: 100% for a theoretical maximum, 80–90% for a healthy battery, 70% or lower for old/cold/high-drain conditions.
- Read the estimated runtime in hours and the human-readable days/hours/minutes breakdown.
About this calculator
This calculator estimates how long a battery will power a device from two numbers you can read off the spec sheet: the battery's rated capacity in milliamp-hours (mAh) and the average current the device draws in milliamps (mA). Because a milliamp-hour is defined as the charge delivered by one milliamp flowing for one hour, dividing capacity by load gives the runtime in hours directly. Real batteries never deliver 100% of their rated charge to the load, so an efficiency factor (typically 70–90%) accounts for voltage regulator losses, the Peukert effect at higher discharge rates, self-discharge, and the fact that devices stop working before the cell is fully empty. Lower the factor for old cells, cold temperatures, or high drain.
How it works — the formula
t (hours) = Capacity (mAh) × η ÷ Load (mA)
where η = efficiency factor (0–1)
C-rate = Load (mA) ÷ Capacity (mAh)A milliamp-hour is a unit of electric charge equal to one milliamp sustained for one hour (Q = I·t). Runtime is therefore charge divided by current. The efficiency factor η scales the rated capacity down to the charge actually delivered to the load under real conditions.
Worked examples
- Inputs:
- capacity=3000, load=150, efficiency=85
- Output:
- 3000 × 0.85 ÷ 150 = 17.00 hours
- Inputs:
- capacity=2000, load=100, efficiency=100
- Output:
- 2000 ÷ 100 = 20.00 hours
- Inputs:
- capacity=5000, load=500, efficiency=80
- Output:
- 5000 × 0.80 ÷ 500 = 8.00 hours
Limitations
- Assumes a constant average load; bursty or duty-cycled loads need an averaged current.
- Does not model the discharge voltage curve — runtime to a device's specific cut-off voltage may differ.
- The Peukert effect is approximated by the single efficiency factor, not modelled per-rate.
A planning estimate, not a guarantee. Verify critical applications (medical, safety) with a measured discharge test.